The invention concerns the technical and mechanical field and more particularly the field for the production and implementation of mechanical parts which, in order to function, need to comprise plastic linings intended to provide a sealing function.
In one preferred, but not exclusive, sphere of application of the invention on the brake circuit for a motor vehicle, the method is known on how to implement inside a hydraulic control cylinder known as a wheel cylinder a piston generally made of metal provided with a plastic or elastomer, such as rubber, sealing lining. This piston most often includes a peripheral throat inside which the elastomer annular-shaped lining is forcefully mounted or forms an extension.
To the extent that they have successfully passed quality controls, if the mounted pistons give full satisfaction concerning their functioning, this production mode does have a certain number of drawbacks which, without still being redhibitory, nevertheless adversely affect performances linked to the use of said piston and to a lesser extent those linked to the production of pistons.
In fact, the placing of the elastomer lining in its housing throat require a temporary deformation of the lining which can result in a local tearing, indeed a full breaking of the lining. Local tears result from damage to the lining when handled or when mounted with a defective tool, or may occur when the elastic deformation limit of the lining is reached during mounting. Thus, the various operations carried out on the lining are likely to render a lining defective which, prior to being produced, conformed with the required quality criteria.
Full tearing occurring during the piston production process results in the damaged lining being unusable and needs to be replaced by a full lining. This loss of lining nevertheless increases the production cost of pistons.
On the other hand, local tears could have more serious consequences. In fact, these tears are not always visible after mounting and the piston bearing a damaged lining may in certain cases successfully pass the various quality controls which are most frequently visible. Thus, the piston with its lining affected by microcracks is likely to be used in the brake circuit of a vehicle. Now, microcracks or tears may result in a rupture of the lining during use and thus a total malfunctioning of the wheel cylinder on which the defective piston is mounted. This hidden defect can therefore have economic and personal consequences much more serious than those resulting from breaking of the lining when the latter is mounted.
Furthermore, it needs to be noted that the various handlings made to the lining between the time it is produced and the time it is mounted also constitute risks of the lining being soiled and polluted by foreign particles likely to affect the proper functioning of the system integrating the mechanical part equipped with the lining.
It thus appears there is a need to have available a new method for producing said pistons making it possible to reduce as much as possible the risk of hidden defects on the lining equipping the piston. It needs to be noted that there is a similar problem concerning all mechanical parts on which it is necessary to use a plastic lining to the extent that mounting of the lining is likely to damage the latter.
So as to resolve said drawbacks, the invention concerns a new method for producing a mechanical part provided with a plastic lining and is able to advantageously reduce the stresses applied to the lining both before and after mounting so as to avoid any risk of said lining deteriorating.
Therefore, the invention concerns a method for producing a mechanical part including a body provided with a plastic moulded lining defining at least one operational contact surface with another mechanical part with respect to which the first mechanical part is mobile.
Moreover, depending on the function of the mechanical part, it may be necessary to define in the moulded lining a recess or chamber. Now known production methods do not make it possible to embody this recess and more particularly so when the lining surrounds the mechanical part and needs to have a radially open chamber. Thus, it seems essential to have available a new moulding method to attain this objective.
So as to attain this objective, the invention concerns a method for producing a mechanical part including a body provided with a plastic lining defining at least one operational contact surface with another mechanical part with respect to which the first mechanical part is mobile.
According to the invention, the method consists of:
using a mould including a main body and at least one secondary core,
placing as the central core the body of the mechanical part in the main body of the mould,
placing the secondary core by at least partially inserting the secondary core in the body of the part forming the central core,
closing the mould,
injecting the plastic material,
opening the mould,
removing the secondary core,
and extracting the mechanical part from the main body of the mould.
According to one characteristic of the invention, the method consists of adjusting the position of the piston of the secondary core with respect to the body to the position of the main body of the mould.
According to another characteristic of the invention, the method consists of using a mould including at least two additional secondary cores nested inside the main body of the mould so that in the mould closed position the position of the secondary cores is adjusted by this nesting.
According to another characteristic of the invention, the method consists of inserting, at least in part, the recess of the secondary cores in a peripheral moulding throat shown by the body of the part forming the central core so as to define an annular chamber in the moulded lining.
According to a further characteristic of the invention, the method consists of fitting in the body of the mechanical part at least one injection channel for passage of the plastic material constituting the lining.
Again, according to another characteristic of the invention, the method consists of using a mould designed in such a way that each functional surface of the lining is formed by one of the elements constituting the mould so as to have functional surfaces without any mould parting line.
The invention also concerns a mechanical part comprising a plastic lining embodied so that the lining is not subjected to stresses at the time it is mounted on the part and exhibits no residual stress after being placed.
So as to attain this objective, the invention also concerns a mechanical part including a body provided with a plastic moulded lining defining at least one functional contact surface with another mechanical part with respect to which the first mechanical part is mobile.
According to the invention, the part is characterised in that the plastic lining is moulded in a peripheral throat shown by the body so as to surround said body and comprises at least one radially open annular chamber.
According to one characteristic of the invention, the peripheral throat has approximately the shape of a radially open xe2x80x9cUxe2x80x9d.
According to another characteristic of the invention, the annular chamber defines at least one sealing lip on the moulded lining.
According to another characteristic of the invention, the body of said part has at least one injection channel fitted so as to allow circulation of the plastic material at the time the lining is moulded.
According to another characteristic of the invention, the mechanical part is used to form a piston in a pneumatic or hydraulic control cylinder.
In one preferred embodiment, the invention concerns a mechanical part intended to form the piston in a control cylinder known as a receiving cylinder for a brake or clutch circuit of a vehicle. The lining then firstly has at least one lip ensuring dynamic sealing, and secondly at least one lip ensuring static sealing.
The invention also concerns a mechanical part intended to form the piston in a control cylinder known as the transmitting cylinder for a brake or clutch circuit of a vehicle. The moulded lining then has firstly at least one functional guiding surface, and secondly at least one lip ensuring dynamic sealing.